Thick film conductors are widely used as a means of interconnecting various passive and active devices for hybrid microcircuits and resistor networks. Utility as a general purpose conductor requires certain performance attributes such as conductivity, solderability, solder leach resistance, compatibility with other circuit components, and ability to be processed under a wide range of conditions. Inherent in the usefulness of thick film conductors is the cost of materials in the composition. It is extremely advantageous to reduce the cost without significantly changing the performance characteristics.
Thick film conductors are comprised of conductive metal and inorganic binder, both of which are in finely divided form and are dispersed in an organic medium. The conductive metal is ordinarily gold, palladium, silver, platinum or mixtures and alloys thereof, the choice of which depends upon the particular combination of performance characteristics which are sought, e.g., resistivity, solderability, solder leach resistance, migration resistance, adhesion and the like.
Thick film techniques are contrasted with thin film techniques which involve deposition of particles by vacuum evaporation or sputtering. Thick film techniques are discussed in Handbook of Materials and Processes for Electronics, C. A. Harper, Editor, McGraw-Hill,. N.Y., 1970, Chapter 12.
In the current economic climate in which noble metals have experienced substantial fluctuations in price, it is especially attractive from a business viewpoint to substitute less expensive base metals as the conductive metal in thick film conductor compositions.
Several base metals have been proposed and used with mixed success as the conductive phase for thick film conductors, among which the most important is copper. However, because of the reactivity of copper metal when it is heated in air, copper-containing thick film conductors are commonly fired in a nonoxidizing atmosphere such as nitrogen. Firing has generally been conducted at 850.degree.-1050.degree. C., a temperature of 900.degree.-1050.degree. C. being preferred. Moreover, it has usually been necessary that any associated functional layer of resistor or dielectric materials also be fired in a nonoxidizing atmosphere to avoid oxidation of the copper-containing layer.
One very important potential application of copper-containing conductors is for the termination of thick film resistors which are usually prepared from ruthenium oxide-based materials such as RuO.sub.2 and ruthenium-containing pyrochlore materials. However, to obtain optimum performance characteristics, such ruthenium-based resistors must be fired in air. When they are fired in nitrogen, currently available ruthenium-based resistor systems frequently do not exhibit good electrical properties. Therefore, it would be advantageous to use dual atmosphere firing in which the ruthenium-based resistor compositions are fired first in air and the copper terminations and conductors are then applied and fired in a nonoxidizing atmosphere such as nitrogen. This technique would facilitate the combination of the excellent electrical characteristics of ruthenium-based resistors with the low cost of copper conductors.